This invention relates to the inhibition of corrosion of structures. The invention has been devised for corrosion inhibition in relation to underground structures, particularly pipework in oil production installations. However, it is to be appreciated that the invention could be applicable more generally, in structures where similar or analogous problems, as described hereafter, arise.
The extraction of oil from underground sources is, in principle, straightforward: a hole is drilled down to an oil bearing stratum in the ground and pipework placed in the hole through which oil can be raised to ground surface level. In some oil wells the oil may be under pressure in the oil-bearing stratum so it flows to the surface without any assistance, but in most cases assistance is required, frequently by the injection of water, through a further pipe, to the oil bearing stratum to displace the oil. The oil then comes to the surface mixed with the water. The water injected to the oil bearing stratum may be sea water, and may be heated so that the oil, if viscous, flows more readily. It will be appreciated that such production techniques produce an environment which is highly conducive to corrosion of steel pipework and components.
The parts of an oil well most prone to corrosion are production zones in which pipework is in contact with the oil-water mixture. The length of the exterior of a well pipe exposed to the mixture is as wide as the production zone. In any well, there may be more than one production zone, the zones being at different depths from one another, and oil production may be switched from one zone to another when the available oil in one zone is depleted. In addition, the inside of the riser pipe which conveys the oil-water mixture to the surface is prone to corrosion.
Corrosion of metals is an electro-chemical process, involving the passage of electrical currents of a greater or lesser magnitude. Where a metal surface is in contact with an electrolyte, differences in potential which arise between different parts of the metal surface, due to metallurgical variations in the material at different places, or local differences in the environment (such as variations in the availability of oxygen at the surface) establish electrochemical cells at which the corrosion process consumes the metal at the anodes. One known technique for inhibiting corrosion is known as cathodic protection, which involves the provision and connection of an external anode to the metal which is to be protected, so that the metal effectively becomes the cathode, and thus does not corrode. The external anode may be a galvanic anode (a metal more reactive than the metal which is to be protected; generally zinc, aluminium, magnesium, or an alloy thereof where it is steel which is to be protected). In this case, the difference in natural potential between the anode and the steel causes an electron flow in the electrolyte from the anode to the steel. At the steel, because the electrical potential between it and the electrolyte solution is, in effect, made more negative by the supply of electrons, corrosive anodic reactions are stifled and only cathodic reactions can take place. The anode or anodes are referred to as sacrificial anodes, as they are consumed in the process.
An alternative protection technique is to employ one or more inert (non-consumable) anodes and use an external source of DC electrical power to impress a current on the anode-cathode system, to achieve the same effect.
In general terms, what is required is to inhibit anodic reactions, either by establishing a zero potential at the surface to be protected or, in conventional cathodic protection, a negative potential which ensures the surface does not become an anode.
Cathodic protection, by the use of sacrificial anodes or by impressed current, is widely used for the protection of structures such as storage tanks, jetties, off shore structures, or reinforced concrete structures where corrosion of the steel reinforcement is a potential problem.
Oil wells present problems so that known cathodic protection systems are not readily applied thereto. Down-well access for the replacement of sacrificial anodes is not possible, while standard impressed-current cathodic protection is not readily applicable. An external anode will only afford protection for a distance along a pipe of not more than two to five pipe diameters, and since the production zone may be moved during the life of a well the establishment of a fixed zone of protection is not useful.
Accordingly, it is the object of the present invention to provide for corrosion inhibition in production zones of oil wells, particularly of the exterior of well pipework, or analogous situations, wherein the above-described disadvantages are overcome or reduced.